Earlier work has indicated that heteropolysacharrides produced by the action of Xanthomonas bacteria on carbohydrate media have potential applications as film forming agents, as thickeners for body building agents in edible products, cosmetic preparations, pharmaceutical vehicles, oil field drilling fluids, fracturing liquids and similar compositions and as emulsifying, stabilizing and sizing agents. Heteropolysacharrides, particularly xanthan gum, have significant potential as a mobility control agent in micellar polymer flooding. Xanthan gum has excellent viscosifying properties at low concentration, is resistant to shear degradation and exhibits only minimal losses in viscosity as a function of temperature, pH, and ionic strength. For these reasons, xanthan gum is an attractive alternative to synthetic polyacrylamides for enhanced oil recovery operations.
Fermentation of the inoculated medium with Xanthomonas organisms for 36 to 72 hours under aerobic conditions results in the formation of xanthan gum which is separated from the other components of the medium by precipitation with acetone or methanol in a known manner. Because of time required to ferment each batch, the low biopolymer content of the fermented medium and the processing required for the recovery and purification of the product, xanthan is relatively expensive.
Other investigators have produced xanthan heteropolysacharrides by means of single stage or multistage "continuous" fermentation. In most instances, Xanthomonas campestris was grown in a medium containing dried distillers solubles (DDS) or other complex nutrient as a source of nitrogen and growth factors. There has been no instance of which we are aware, however, that xanthan was produced by single stage continuous fermentation with Xanthomonas campestris using a chemically defined synthetic medium containing only glucose, ammonium chloride as the sole nitrogen source, and mineral salts.
It is well known that the continuous production of xanthan has been hampered by a tendency of the culture (Xanthomonas campestris) to change or degenerate after a fairly short and specific number of turnovers, i.e., the time required to completely replace one volume of broth in the fermentation vessel, or the reciprocal of the dilution rate. Normally, 6 to 9 turnovers are the maximum that can be obtained before degeneration of the culture occurs. At the same time, there is a decrease in viscosity, a loss in volumetric productivity of xanthan, i.e., grams of xanthan per liter of broth per hour, and the appearance of a variety of culture variants or strains that no longer produce xanthan or else produce a poor quality of xanthan. It has been demonstrated that this phenomenon occurs when DDS is used as the complex nitrogen source, whether in the whole form or as a water soluble extract.
The most pertinent publications of which we are aware are as follows:
1. P. Rogovin et al,. 1970, "Continuous Fermentation to Produce Xanthan Biopolymers: Laboratory Investigation," Biotechnol. Bioeng., XII, pages 75-83.
2. K. W. Silman, et al., 1972, "Continuous Fermentation to Produce Xanthan Biopolymer: Effect of Dilution Rate," Biotechnol. Bioeng., XIV, pages 23-31.
3. P. Rogovin, U.S. Pat. No. 3,485,719, Continuous Production of Xanthan.
4. G. P. Lindblom, et al., U.S. Pat. No. 3,328,262, Heteropolysacharride Fermentation Process.
5. Netherlands patent application No. 7,612,448, Method for the Production of Bacterial Polysacharrides.
6. "Production of Polysaccharides by Xanthomonas campestris in Continuous Culture," FEMS Microbiology Letters 3 347-349 (1978) by I. W. Davidson.
7. Process for the Production of Xanthan Gum, British patent application No. 2,008,138A (Tate and Lyle Ltd.)